Unforgiven was watched by 1.7 million viewers (12.5% audience share), with another 100,000 watching an hour later on ITV1 +1, compared with CSI: Miami's audience of 2 million (8.7%).

CSI:Miami was just behind Channel 4's Seven Dwarves, which scored 2.1 million (8.8%), with another 200,000 watching it later on Channel 4 +1.

However, the slot was won by BBC'1's Harry's Arctic Heroes with 4.5 million (19%), followed by film The Boy in the Striped Pyjamas on BBC2, which drew 3.3 million (14%) – including the BBC HD simulcast.

One source pointed out: "It may be summer but some advertisers may be wondering why they're paying ITV1 rates when Channel 5 is just behind it."

However, with the recent successful return of The X Factor, which on Saturday averaged 10.1 million viewers, and new Ant and Dec gameshow Red or Black and a new Jonathan Ross chat show premiering this weekend, ITV1 is expected to bounce back.

An ITV spokesman said: "The summer months often see lower viewing levels across all channels. ITV1 has had a terrific first half of 2011 broadcasting the most watched programmes in drama and soap, entertainment, and sport as well as launching eight out of the top 10 new dramas on any channel and last week was crowned the MGEITF [MediaGuardian Edinburgh International Television Festival] terrestrial channel of the year."

• To contact the MediaGuardian news desk email editor@mediatheguardian.com or phone 020 3353 3857. For all other inquiries please call the main Guardian switchboard on 020 3353 2000. If you are writing a comment for publication, please mark clearly "for publication".

• To get the latest media news to your desktop or mobile, follow MediaGuardian on Twitter and Facebook.